I had not noticed the word "flat" in any definition mainly it is defined as A circle is a shape consisting of a curved line.
A circle, (Euclidean) geometrically speaking, is "two dimensional".
Any "circle" contingent upon 3+ dimensions, which likely includes any external sense objects are representative and not actually circles such as with the given image.
Why? because they have "thickness",
such as with a cylinder.
Circles are curious things though, because their "length" is their "width" which would seem to make them one, not two, dimensional.
But Feists and abstraction don't tend to get on well, so this is likely a waste of time; however, it does serve to show the folly of straying from the true Liberal Arts.